Cellular communications systems providing a telephony service are attracting increasing commercial interest. In such systems a number of mobile or portable telephone instruments are in radio communication with a local base station. In a typical system, traffic is carried on the radio link between the telephone instrument and the base station in a digitally encoded format. This provides security of transmission and some resistance to interference. Because of the high (UHF) radio frequencies used for transmission between a mobile telephone and the base station, this transmission is effectively restricted to line of sight.
In metropolitan or dense urban areas the direct line of sight between a mobile telephone and the base station may be blocked by buildings and the mobile telephone thus receives a distorted version of the transmitted signal comprising a number of indirect multipaths or reflected signals. These signals may have relative time delays of more than 5 microseconds thus causing significant overlap of the various indirect signals. This can cause effective loss or `break up` of the received signal.
A further difficulty, particularly in dense urban areas, is that of spectrum allocation. Indeed, the availability of spectrum can restrict the number of users that can be accommodated.
A recent approach to these problems has been the introduction of the code division multiplex (CDM) transmission technique. This technique is described, for example, by U Grob in IEEE Journal Selected Areas Comms., 8, June 1990 pp 772-780 and by W C Y Lee in IEEE Transactions on Vehicular Technology, 40 No 2, 2nd May 1991 pp 291-302.
In a CDM transmitter a number of data sources produce information bit streams in parallel. Each bit stream is associated with a corresponding binary spreading sequence which spreading sequence repeats after a predetermined period. The spread bit streams from the source are modulated onto a carrier and then added linearly to give a multiband composite RF signal which is broadcast to the mobile telephones within the service area of the transmitter. At each receiver, the composite signal is decomposed to individual messages by correlation with a particular spreading sequence associated with a particular message.
The technique allows a number of users to occupy the same channel thus significantly increasing the effective spectrum availability. However, with the rapidly increasing popularity of mobile communications services, there is a need to accommodate further users within each channel.
It is an object of the invention to minimise or to overcome this disadvantage.
A further object of the invention is to provide a receiver configuration that permits an increased number of users to occupy the same channel.